1. Field of the Invention
The present invention relates to a laser irradiation device, and more particularly to a laser irradiation device irradiating a linear laser beam whose length in the major axis can be altered using a lens optical system.
2. Description of the Related Art
Recently, a liquid crystal display (hereinafter, referred to as an "LCD") has been developed using polysilicon (hereinafter, referred to as "p-Si") as a material of switching elements, although amorphous silicon (hereinafter, referred to as "a-Si") was widely used before. In order to form or promote the grain growth of the p-Si, an annealing process using a laser beam is adopted.
FIG. 1 is a conceptual diagram showing a laser irradiation device for performing the laser annealing.
In FIG. 1, numeral 1 denotes a laser generator, numerals 2 and 11 denote reflection mirrors, numerals 3, 4, 5 and 6 denote cylindrical lenses, and numerals 7, 8, 9, 12 and 13 denote condenser lenses. Numeral 10 is a slit that defines the linear width direction of the laser beam irradiated on a substrate. A stage 14 supports the substrate 20 to be processed, on which an a-Si film is formed, and can be moved in the x and y directions.
The laser beam irradiated from the laser generator 1 has an energy distribution in the beam plane. The difference of the energy levels in the beam plane brings about a difference of annealing condition for the a-Si, which generates a variation of the film quality of the p-Si formed by the polycrystalline process depending on the location in the p-Si film. In order to eliminate such a defect, the energy level of the laser beam should be as flat as possible in the beam plane. In the device shown in FIG. 1, the laser beam is split in the horizontal and the vertical directions with respect to the optical center using a pair of cylindrical lenses 3 and 5, as well as a pair of cylindrical lenses 4 and 6. The vertically split beams are condensed by the condenser lenses 8, 9, 12 and 13 and focused in one direction (the vertical direction) as shown in FIG. 2. Thus, the energy variation of the beam is canceled in the vertical direction. At the same time, the horizontally split laser beams are condensed by the condenser lens 7 so as to cancel the energy variation and enlarged in one direction (the horizontal direction) as shown in FIG. 3. In this way, passing the cylindrical lenses 3-6, the condenser lenses 7-9, 12 and 13, the laser beam is focused in one direction and enlarged in the other direction in the irradiation plane. Thus, the laser beam becomes a linear laser beam having substantially uniform energy level in each direction in the plane, which is irradiated toward the substrate 20 to be processed. The stage 14 on which the substrate 20 is placed can be scanned by the irradiated linear laser in the linear width direction. By this scanning in the linear width direction, the annealing process can be performed for a large area. Thus, laser annealing process with high throughput can be achieved by this device.
Here, the linear longitudinal direction means the direction of the major axis of the laser beam, while the linear width direction means the direction of the minor axis of the laser beam.
FIG. 4 shows laser beam irradiation toward a mother glass substrate 30 for making multiple TFT substrates 31 on which TFT's including p-Si as an active layer are arranged, as an LCD panel on the large glass substrate 30.
On the mother glass substrate 30, a-Si is deposited and the linear laser beams 32 and 33 are irradiated scanning in the scan direction, thereby causing the a-Si to be polycrystallized to become p-Si.
However, in the conventional laser irradiation device the optical system is fixed, so the area of the linear laser beam irradiated on the substrate is fixed. Especially, since the length of the linear laser beam irradiated on the substrate in the linear longitudinal direction (the major axis direction) is fixed, there is a problem as explained below.
Each size of the plural TFT substrates 31 formed on one mother substrate 30 is determined as desired, e.g., as 2.5 inches or 3 inches by diagonal size. Therefore, if the size of the mother glass substrate, which is a to-be-processed substrate 20, or the size of the TFT substrate formed on the mother substrate changes, the laser irradiation device cannot be used anymore, which is not rational from the viewpoint of cost.
In addition, if a laser irradiation device having improper length of the linear line is used, when irradiating the laser beam toward the mother glass substrate 30 on which the TFT substrates 31 are arranged as shown in FIG. 4, for example, the whole surface of the fourth TFT substrate 31 cannot be irradiated from above in the figure by the first scan of the laser beam 32. Therefore, the fourth TFT substrates 31 is required to be irradiated by the second laser beam irradiation 33 overlaying the first irradiated area. Then, an area 35 is generated that is irradiated by both the first and second scans of the laser beam on the fourth TFT substrate 31. In this case, since the laser irradiation condition is different between the area 35 and other area, the grain size of the polycrystalline semiconductor film may be uneven within the fourth TFT substrate 31. In addition, characteristics of each TFT substrate 31 on the mother glass substrate 30 may be varied.
In order to prevent each of the irradiated laser beams from overlaying each other on a certain TFT substrate, a space 35 may be disposed for each block of the TFT substrates corresponding to the length in the major axis of the linear laser beam as shown in FIG. 5. However, this measure may cause another problem such that limitation in the size of each substrate would occur, for example, the mother substrate is required to be enlarged, or the size of the TFT substrate should be decreased. At the same time, this measure can cause decreasing number of TFT substrates that can be produced from one mother substrate, and increase of the manufacturing cost.